A speech coding technology that compresses a speech signal at a low bit rate is important for efficiently using a radio wave etc. in mobile communication. Further, in recent years, expectation for improvement of quality of communication speech has been increased, and it is desired to implement communication services with high realistic quality. Here, realistic quality means the sound environment surrounding the speaker (for example, BGM), and it is preferable that signals other than a speech signal such as audio can be coded with high quality.
There are schemes such as G726 and G729 defined in ITU-T (International Telecommunication Union Telecommunication Standardization Sector) for speech coding of coding speech signals. In these schemes, coding is carried out at 8 kbit/s to 32 kbit/s targeting a narrow band signal (300 Hz to 3.4 kHz). Though these schemes are capable of coding at a low bit rate, since the targeted narrow band signal is narrow up to a maximum of 3.4 kHz, this quality tends to lack realistic quality.
Further, in ITU-T and 3GPP (The 3rd Generation Partnership Project), there are standard schemes of speech coding with signal band of 50 Hz to 7 kHz (G.722, G.722.1, AMR-WB, and the like). Though these schemes are capable of coding a wideband speech signal at a bit rate of 6.6 kbit/s to 64 kbit/s, it is necessary to increase bit rates relatively for coding wideband speech with high quality. From the viewpoint of speech quality, wideband speech is high quality compared to narrow band speech, but it is difficult to say that this is sufficient for services requiring high realistic quality.
Typically, when maximum frequency of a signal is 10 to 15 kHz, realistic quality equivalent to FM radio quality can be obtained, and, when maximum frequency is 20 kHz, quality equivalent to CD can be obtained. Audio coding such as a layer 3 scheme or AAC scheme defined by MPEG (Moving Picture Expert Group) is suitable for a signal having such band. However, when these audio coding schemes are applied as a coding scheme for speech communication, it is necessary to set a high bit rate in order to code speech with good quality. There are also other problems such as a problem that a coding delay becomes substantial.
As a method of coding a signal with wide frequency band at a low bit rate with high quality, there is a technology for reducing overall bit rate by dividing the spectrum of an input signal into low frequency band and high frequency band to obtain two spectrums, duplicating the low frequency band spectrum and substituting the low frequency band spectrum for the high frequency band spectrum (using the low frequency band spectrum in place of the high frequency band spectrum) (for example, refer to Patent Document 1). In this technology, a large number of bits are allocated for coding of the low frequency band spectrum, and coding is performed with high quality, while on the other hand, the high frequency band spectrum duplicates the coded low frequency band spectrum as basic processing, and coding is performed with a small number of bits.
Further, as a technology similar to this technology, there are a technology of improving quality by performing approximation on band where coded bits cannot be sufficiently allocated using other predetermined partial band spectrum information (for example, refer to Patent Document 2), and a technology of duplicating a low frequency band spectrum of a narrow band signal as a high frequency band spectrum as basic processing in order to extend band of a narrow band signal to a wideband signal without additional information (for example, refer to Patent Document 3).
In either technology, another band spectrum is duplicated for band where it is wished to compensate a spectrum, and after gain is adjusted to smooth the spectrum envelope, this duplicated spectrum is inserted.    Patent Document 1: Japanese Patent Publication Laid-open No. 2001-521648.    Patent Document 2: Japanese Patent Application Laid-open No. HEI9-153811.    Patent Document 3: Japanese Patent Application Laid-open No. HEI9-90992.